Ink cartridges are completely transparent, unlike recent genuine Canon cartridges. This makes it easy to check the ink levels and there's less chance of overfilling the tanks and making a mess (you can see what you're doing).
The important feature of refillable ink cartridges is that the fill holes/plugs are recessed into the top, so the plugs don't cause interference in late model printers. Unless you've tried refilling original Canon PGI-520, CLI-521, PGI-525, CLI-526, or similar ink cartridges...and ended up with few plugs knocked out inside the printer - the ink tanks leaking out and a major mess to clean up...you may not see the significance in this.
Another important feature of refillable ink cartridges is the "ink level" chip. Refillable Canon ink cartridges come fitted with ARC (auto-reset chips).
ARC chips reset automatically - when a cartridge is removed from the printer. The reset technology is built into the ARC chips, so you don't need a chip resetter. Very Clever stuff! On the negative side...in some models the ARC chips also reset, when the printer is switched off. So - unless you always leave your printer powered on, ARC ink levels may reset (to full) even if you don't refill them, so it is more difficult to keep track of the actual ink levels.